1. Field of the Invention
This invention relates to a polarization beam splitting optical system and a projection type display optical system to be used in a projection type image display apparatus, which guides light using a polarization splitting film and projects the image of an original image formed on an image forming element.
2. Description of Related Art
As an example of a projection type image display apparatus, which projects and displays an image on a large screen, there is a display apparatus which makes use of a reflection type liquid crystal display element (reflection type image forming element). With this projection type image display apparatus, light (illumination light), which is made incident on a reflection type liquid crystal display element, is modulated in accordance with an original image displayed on the reflection type liquid crystal display element and is then reflected, and this modulated and reflected light (image light) is magnified and projected onto a screen by means of a projection optical system.
With such a projection type image display apparatus, there are cases where a polarization beam splitter is used to guide the illumination light to the reflection type liquid crystal display element and furthermore guide the image light, reflected by the reflection type liquid crystal display element, to the projection optical system.
Here, the polarization beam splitter provides both the action of a polarizer that prepares illumination light of a specific polarization component and the action of an analyzer that prepares image light of a specific polarization component.
As an example of an optical system of a projection type image display apparatus that uses a polarization beam splitter, there is that which is disclosed in Japanese Patent Publication No. H7(1995)-38050 (corresponding to U.S. Pat. No. 5,327,270). As shown in FIG. 31, with the optical system disclosed in this publication, a ¼-wave plate 103 is disposed between a polarization beam splitter 101 and a reflection type liquid crystal display element 102. And for a light ray 111, for which the polarization direction 113 after reflection by a polarization splitting film 104 of the polarization beam splitter 101 becomes inclined with respect to the X-axis direction in FIG. 31, the phase advance axis 105 of the ¼-wave plate 103 is set in the X-axis direction 105. The light ray 111 is thereby made to pass twice in a round-trip manner through the ¼-wave plate 103 before and after reflection by the reflection type liquid crystal display element 102 and the polarization direction 113 that is inclined with respect to the X-axis is thereby inverted with respect to the X-axis and made to coincide with the S-polarization direction 114 at the polarization splitting film 104 of the reflected light 112 reflected by the reflection type liquid crystal display element 102.
If the brightness of the projected image is to be made brighter, the F number of the illumination system must be made brighter and thus the angle of incidence of light onto the polarization beam splitter must be made greater. However, this may lower the analyzing performance of the polarization beam splitter.
Here, the F number (Fno) of the illumination system is related to the convergence angle ψ of the illumination light flux that is converged onto an arbitrary point on the reflection type liquid crystal display element and is defined as:Fno=1/(2 tan ψ).
The problems that arise when the Fno of the illumination system is made brighter (less) shall now be described using FIG. 21.
With the prior-art optical system shown in FIG. 31, only the light rays that are parallel to the X-Y plane of the coordinate system, having the center of the polarization beam splitter 101 (polarization splitting film 104) as the origin, are taken into account as the light rays made incident on polarization beam splitter 101. However, with an actual illumination light flux, light rays are incident on the polarization beam splitter 101 from various directions centered about the Y-axis (optical axis) (at various angles with respect to the Y-axis) as shown in FIG. 21.
The inclinations of polarization directions in the light rays that are emerged from the polarization beam splitter 101 in this case can be expressed as shown in FIG. 22. y indicates the direction of a light ray that is incident on the incidence surface i (shown in FIG. 21) of the polarization beam splitter from the direction parallel to the Y-Z plane and x indicates the direction of a light ray that is incident on the incidence surface i from the direction parallel to the X-Y plane. Also, A indicates the direction of a light ray that is made incident from the direction parallel to a plane inclined by +45° with respect to the Y-Z plane and B indicates the direction of a light ray that is made incident from the direction parallel to a plane inclined by −45° with respect to the Y-Z plane. Hereinafter, the directions of A and B shall be referred to as “diagonal directions of the illumination light flux.”
As shown in FIG. 22, the light rays besides those of the y direction are all in polarization states in which the polarization directions are inclined with respect to the x direction in accordance with the S-polarization directions of the respective light rays at the polarization splitting film 104. Also, even for the same A direction, light rays a1 and a2, which differ in the incidence angles onto the polarization splitting film, differ in the polarization inclination angles γa1 and γa2 as indicated by the following equations (the same holds for the polarization inclination angles γb1 and γb2 of the incident rays b1 and b2 from the B direction).γa1=−γb1≠γa2γa2=−γb2≠γa1
In FIG. 23, for the light ray a1, the polarization state after emergence from the polarization beam splitter 101 is indicated as P1 and the polarization state after round-trip passage through the ¼-wave plate 103, shown in FIG. 21, is indicated as p2.
The polarization directions of the polarization states p1 and p2 are in an inverted relationship with respect to the x-axis due to the action of the ¼-wave plate 103. Here, since the light ray a1 is reflected by the reflection type liquid crystal display element 102, when it is incident on the polarization beam splitter 101 again, it will be light that proceeds along the same optical path as but in reverse of the optical path of the light ray a2 that is emerged from the polarization beam splitter 101.
The S-polarization direction at the polarization splitting film 104 at this point is the direction of polarization inclination of the light ray a2 in FIG. 22, and this direction is indicated by a dotted line in FIG. 23.
As can be understood from FIG. 23, a deviation A arises between the polarization direction p2, resulting from conversion by the ¼-wave plate 103, and the S-polarization direction at the polarization splitting film 104, and even if the polarization splitting performance of the polarization splitting film 104 is ideal, a light ray of a diagonal direction (the direction of A or B) of the illumination light flux will not be analyzed completely.
This deviation A becomes more significant the greater the angle ψ with respect to the Y-axis (optical axis of the illumination light flux) at the incidence surface i of the polarization beam splitter in FIG. 21, that is, the brighter the Fno of the illumination system. The amount of leakage light at the polarization beam splitter 101 is thus increased, and as the brightness of a projected image is made brighter, the contrast is lowered further.
Here, since the S-polarized components are removed by the polarization splitting film, the polarization direction of the leakage light is the direction of the P-polarization component m1 at the polarization splitting film as shown in FIG. 24. In order to compensate for the characteristics of the polarization beam splitter, an analyzing element, such as a polarizing plate having a transmission axis in the direction of the Y-Z plane, may be added to the emergence side of the polarization beam splitter 101. However, since the oscillation direction of the leakage light (P-polarization component) m1 is substantially the same as the direction (y direction) of the transmission axis of the polarizing plate 105, most of the leakage light m2 will remain as shown in FIG. 25. Thus the leakage light described here will necessarily reach the screen if it is emerged from the polarization beam splitter.